There are many applications in which highly stable oscillators are required, both in integrated circuit systems and in discrete systems. For example, in integrated circuit systems, highly stable oscillators are required for counters, frequency dividers, frequency multipliers, phase locked loops, charge pumps, and any other circuit which requires a constant clock frequency. Ring oscillator circuits frequently are used to provide the clock frequency in integrated circuit systems because ring oscillators lend themselves to fabrication using integrated circuit techniques. When a ring oscillator circuit is used to provide a time base in a digital system, the operating frequency of the ring oscillator circuit must be free from frequency drift caused for by variations in temperature or supply voltage. Various methods have been used to minimize the effects of temperature and supply voltage variations on circuit operation, including the use of complementary circuit stages so changes due to temperature and voltage variations offset one another. There is a need for a ring oscillator circuit formed by integrated circuit techniques and that includes a reference signal source that operates independently of variations in temperature.
A further consideration is that for oscillator circuits and other time base generating circuits that are formed by integrated circuit techniques, the operating frequency must be established during batch processing by producing a component, such as a resistance, of the oscillator circuit to have the value that is required to provide the desired operating frequency. Because the component values of integrated circuit devices are determined by the masks that are used in the production of the integrated circuit devices, the component values are fixed once the mask has been designed. Moreover, characteristics, such as the operating frequency of an oscillator formed on an integrated circuit device, cannot be verified until after batch processing and passivation of the integrated circuit device. Consequently, if after testing it is found that the operating frequency of such oscillator is not the desired frequency, the integrated circuit device which includes the oscillator cannot be used and design of the integrated circuit device has to be altered, for example, by taping out the mask to allow production of integrated circuit devices in which the oscillator circuit has the proper operating frequency. This results in increased production time and increased costs. Thus, there is a need to be able to adjust the frequency of an integrated circuit oscillator after fabrication and passivation of the integrated circuit oscillator.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an oscillator circuit that can be formed as an integrated circuit device, and which includes a reference signal source, the operation of which is independent of variations in temperature, and wherein the operating frequency of the integrated circuit oscillator can be adjusted in a simple manner after fabrication and passivation of the integrated circuit oscillators.